Improvement of the initial placement of modifier meshes:

Sphere and Cylinder are scaled to the same volume as Box
Newly entered modifier meshes are rotated parallell to the world coordinates.
If the instance coordinate system is rotated and scaled, it is not possible
to create an unskewed modifier to world transformation. In that case
the best possible transformation is found to minimize least squares error
of the 8 corners of the new modifier mesh bounding box using
Levenberg-Marquardt algorithm.

FIXME:
1) The Levenberg-Marquardt non-linear least squares does not converge
nicely, it may require some tuning.
2) Above all, if 1) is called, then often the skew of the modifier mesh
is so high, that it is likely more useful to display the modifier
with zero rotation and inverse scaling, so that the modifier will be
of correct size, but not parallel to the world coordinates.

src/libslic3r/Geometry.hpp

@@ -242,6 +242,7 @@ public:
 
     void set_scaling_factor(const Vec3d& scaling_factor);
     void set_scaling_factor(Axis axis, double scaling_factor);
+    bool is_scaling_uniform() const { return std::abs(m_scaling_factor.x() - m_scaling_factor.y()) < 1e-8 && std::abs(m_scaling_factor.x() - m_scaling_factor.z()) < 1e-8; }
 
     const Vec3d& get_mirror() const { return m_mirror; }
     double get_mirror(Axis axis) const { return m_mirror(axis); }